Since 2006, the number of reported US listeriosis outbreaks associated with cheese made under unsanitary conditions has increased. Two-thirds were linked to Latin-style soft cheese, often affecting pregnant Hispanic women and their newborns. Adherence to pasteurization protocols and sanitation measures to avoid contamination after pasteurization can reduce future outbreaks.
Listeria monocytogenes is a widely distributed environmental bacterium that can grow at refrigeration temperatures. Infection can cause severe illness and death. Persons at higher risk for infection include older adults, persons with weakened immune systems, and pregnant women and their newborns.
Listeriosis outbreaks have been associated with refrigerated ready-to-eat foods, including hot dogs, delicatessen meats, soft cheeses, milk, and other dairy products. For soft-ripened cheeses, the risk for listeriosis per serving is estimated to be 50- to 160-fold greater for cheese made from unpasteurized milk than pasteurized milk (1). Pasteurization kills L. monocytogenes; however, milk labeled as pasteurized and dairy products made from pasteurized milk can become contaminated due to inadequate hygiene practices after pasteurization. The earliest reported listeriosis outbreak in the United States in 1985, associated with Latin-style cheese (in particular, queso fresco and cotija), resulted in 142 illnesses, 28 deaths, and 20 fetal losses (2). Although the cheese was labeled as made from pasteurized milk, raw milk was inadvertently introduced into the pasteurized milk.
A US retail survey of several soft cheeses (Latin-style, blue-veined, mold-ripened) from 2000–2001 detected L. monocytogenes in 1.3% of cheeses made from unlabeled or unpasteurized milk and 0.5% of cheeses from pasteurized milk (3). However, pasteurized-milk cheese is much more commonly consumed than cheese made from unpasteurized milk. In a survey of food exposures conducted in 10 US states during 2006–2007, respondents reported eating types of soft cheeses (15.3% for blue-veined cheese, 6%–11% for other soft cheeses; pasteurization status unknown) more frequently than they reported eating cheeses made from unpasteurized milk in the previous 7 days (1.6%) (4). We describe outbreaks linked to soft cheese (both soft-ripened and acid-coagulated–ripened cheeses), demographic characteristics of the persons affected, and possible contributing factors to help inform prevention messaging for persons at higher risk.